1. Field of the Invention
The present invention relates to a three-dimensional shape measuring device for measuring a three-dimensional shape of an object using the principles of triangulation, and a sensor employed for the measurement.
2. Description of the Related Art
A conventional procedure of measuring a three-dimensional shape of an object using the light-section method will be described in conjunction with FIG. 7. A mirror 43 is arranged forward of a laser source 41 that emits plane light, and an object 44 to be measured is placed forward of the mirror 43. An image sensor 45 is opposed to the object 44.
The laser source 41 emits plane light. Then, the plane light reflected from the mirror 43 irradiates the object 44 to create a bright line 46 on the surface of the object 44. An image 47 of the bright line 46 is projected on the image sensor 45 by a condenser that is not shown. For example, a point 49 on the bright line 46 corresponds to a point 50 of the image 47 on the image sensor 45. An equation representing the plane light 42 for irradiating the object 44 is determined with a deflection angle of the mirror 43. On the other hand, the point 49 on the bright line 46 lies on a straight line 51 linking the point 50 on the image 47 and the center point 52 of the condenser. An equation representing the straight line 51 is determined with the coordinates of a pixel 48 of the image sensor 45 corresponding to the point 50 of the image 47 and the coordinates of the center point 52 of the condenser. Then, the simultaneous equations of the equation representing the plane light 42 and that representing the straight light 51 are solved to calculate the three-dimensional coordinates the intersection 49 between the plane light 42 and straight line 51. Similarly, coordinates of points on the bright line 46 are calculated according to the resolution of the image sensor 45.
When the mirror 43 is rotated to move the plane light 42 for irradiating the object 44, the bright line 46 appearing on the surface of the object 44 moves on the surface. FIG. 7 plots the bright line 46 moving on the surface of the object 44, and the image 47 moving on the image sensor 45 in synchronization with the movement of the bright line 46. Then, the angle of the mirror 43 is varied incrementally. Every time the mirror 43 changes its angle, the coordinates of points on the bright line 46 appearing on the surface of the object 44 are calculated. Thus, three-dimensional coordinates constituting the whole object 44 are provided.
The three-dimensional shape of the object 44 can be measured as mentioned above. However, to obtain the three-dimensional coordinates of the whole object 44, several tens or hundreds of images 47 must be acquired using the image sensor 45. Consequently, it takes too much time to measure the coordinates constituting the whole object 44. Real-time processing is difficult to achieve at a rate of 30 frames per second.
To shorten the processing time, U.S. Pat. No. 4,794,262 has proposed a new procedure based on the light-section method shown in FIG. 8. A laser beam emitted from a laser oscillator 61 is expanded in the Form of a plane by a lens 62, then reflected from a polygon mirror 63 rotating at a certain angular speed to irradiate an object 64. An image of a bright line appearing on the object 64 is formed on an image surface 71 of an image sensor 70 in an imaging unit 69. A photosensor 65 is placed in the vicinity of the polygon mirror 63. When the rotation angle of the polygon mirror 63 comes to meet a reference value, light reflected from the polygon mirror 63 enters the photosensor 65. The photosensor 65 outputs a reset signal to a timer 67 and to a counter 68 respectively. Thereby, the timer 67 and counter 68 start up. The timer 67 generates clock pulses and the counter 68 counts the clock pulses. That is to say, an angle of deviation of the polygon mirror 63 from the reference angle is represented as a duration or an output of the counter 68. Therefore, an equation representing plane light can be satisfied with the output value of the counter 68.
When a bright line image crosses pixels on the image plane 71 of the image sensor 70, an output value the counter 68 provided at that time is latched and put in a memory unit 72. Similarly, before the plane light completes a sequence of scanning the surface of the object 64, durations for pixels are put in the memory unit 72 of the imaging unit 69 consecutively. Then, when the scanning is completed, the values of the durations are read from the memory unit 72 sequentially. A data processor 73 calculates coordinates of points on the surface of the object 64 corresponding to pixels. Thus, three-dimensional coordinates constituting the whole object 64 are provided.
In the method shown in FIG. 8, the whole object 64 can be measured merely by scanning plane light over it once. This results in greatly-reduced measurement time.
However, to improve precision in shape measurement, a minimum unit for the clock pulses that the timer 67 generates must be reduced to upgrade resolution of time measurement. For example, the whole scanning time required for a sequence of scanning plane light should be resolved into 4096=2.sup.12 durations rather than 2048=2.sup.11 durations. Higher resolution results in finer durations, thus realizing high-precision shape measurement. Then, in order to resolve the whole scanning time into 4096 durations and measure the durations, lines each capable of transmitting at least 12 bits must tie connected to the counter 68 in association with the pixels of the image sensor 70. Thereby, durations are fetched as the outputs of the counter 68. Then, an LSI wiring technology must be employed to connect such lines between pixels. Thereby, an area occupied by the lines must be expanded, while an area occupied by the pixels must be shrunk. That is to say, when an effort is made to improve precision in time measurement, the numerical aperture of image sensor deteriorates. Consequently, a bright line image cannot be acquired with high precision.